Wood burning down draft stove with correlated temperature



May 31, 1955 A. B. BOWER ET AL 2,709,551

woou BURNING DOWN DRAFT STOVE WITH CORRELATED TEMPERATURE CONTROLLED PRIMARY AND SECONDARY AIR SUPPLY Filed April 2, 1954 IIIIIIIIIIIIIIIIIIIA I N VENT ORS [lien/100?! Alla/v11. J wen w W 'W ATTORNEYS United States Patent WOOD BURNING DOW'N DRAFT STOVE WITH CORRELATED TEMPERATURE CONTROLLED PRIMARY AND SECONDARY AIR SUPPLY Albert B. Bower and Allan R. Spreen, Richmond, Va., assignors to Southern Steel and Stove Company, Inc., Richmond, Va., a corporation of Virginia Application April 2, 1954, Serial No. 420,562

4 Claims. (Cl. 236-96) This invention relates to wood burning down draft stoves of that type in which the down draft is controlled by a thermostatically governed damper generally located in the down draft pipe, the thermostat being so positioned as to respond predominantly to variations in stove temperature. Thus, the thermostat is set to close the damper .15

progressively as the heat of combustion rises, and to open the damper to progressively stimulate combustion as the fire burns low. At the peak of combustion the damper will be closed or substantially so, the supply of oxygen to the fire being cut oil? and the damper remaining closed until the intensity of combustion has diminished to the point at which the thermostat is set to open it.

The conditions within a down draft wood burning stove of the type described, after the down draft damper has been closed are about as follows. There is sufficient oxygen in the stove to support only a limited combustion. The stove atmosphere may be regarded as being divided into three layers with respect to the amount of oxygen therein contained. At the bottom, surrounding the wood, is a layer which receives a little oxygen drawn in as leakage past the closed damper, or a little more if the damper is not quite closed, but in any event, there is insufficient oxygen to support complete combustion. Since wood is not uniformly combustible, the action of the small amount of oxygen present is selective, so that over a great part of the fuel combustion has become extinguished, the burning being confined to the more intensely heated regions where pieces lie close, and to pockets of volatile matter which may burn as gas flames. A considerable amount of volatile gas is distilled from the wood without burning, or being imperfectly burned, which forms an intermediate layer overlying the wood.

The uppermost layer consists of the products of imperfect combustion, but receives a little oxygen through diffusion from the atmosphere in the chimney stack pipe, it being borne in mind that the atmosphere of the stove is static while the down draft damper is closed, that is, there is substantially no draft up the chimney stack.

Now it may happen that a gas flame will suddenly shoot out from the smoldering wood, agitating the stove atmosphere to the extent that enough oxygen from the uppermost layer becomes mixed with the volatile gases constituting the intermediate layer, to render the latter explosive. This explosive mixture is ignited by the gas flame, and produces an explosion, generally mild, due to dearth of oxygen, but sufiiciently forceful to drive soot, smoke and gases through every crack and cranny of the stove and into the room.

The above is thought to be a correct and plausible explanation of what causes frequent similar explosions in down draft wood burning stoves when the down draft damper has been closed some little time, and which constitutes a serious objection to the use of such stoves.

The principal object of the present invention is to provide a secondary air supply for down draft wood burn mg stoves of the type described, entering the stove immediately above the zone of primary combustion, that is,

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some distance above the point at which the down draft pipe communicates with the combustion chamber, and below the stack pipe opening, said secondary air supply being controlled by a damper thermostatically governed responsive to stove temperature, and correlated with the down draft damper to open progressively as the latter closes, and vice versa, whereby the gaseous matter volatiiized by the heat of waning combustion will at all times be supplied with sufficient oxygen to be normally consumed.

Other objects of the invention will appear as the following description of a practical embodiment thereof proceeds.

In the drawing which accompanies and forms a part of the following specification, and throughout the figures of which the same reference characters have been used to denote identical parts:

Figure l is a perspective view of a down draft wood burning stove embodying the principles of the invention;

Figure 2 is a section through the thermostatic damper control of the secondary air supply, taken along the line 2-2 of Figure 1;

Figure 3 is a plan view of the thermostatically controlled damper shown in Figure 2;

Figure 4 is a perspective view of the thermostatically controlled down draft damper.

Referring now in detail to the several figures, the numeral 1 represents a down draft wood burning stove comprising a drum or body 2, which in the form shown is oval, having a continuous side wall and a top and bottom. The drum 2 has an opening 3 at one end for feeding fuel, the bottom of the opening being a considerable distance above the bottom of the stove. The opening 3 is framed about by an outwardly projecting snout 4, having an opening in the front closed by the fuel door 5.

A down draft pipe 6 also extends from said end of the drum, being offset with respect to the snout 4, but adjacent thereto. That part of the drum 2 below the level of the bottom of the opening 3 may be considered the fuel chamber. The down draft pipe communicates with the lower part of the the] chamber by means of the elbow 7. A damper 8 is mounted in the down draft pipe at its upper end and controls the draft to the fuel chamber. The damper is operated by a thermostat comprising a bimetallic spiral 10 connected to one end to an adjusting knob 11, and at the other end to the shaft of the damper, said coil being so arranged that as it heats it moves the damper in a closing direction. Due to the proximity of the thermostat 9 to the snout 4, it is predominantly affected by the temperature of the stove rather than by room temperature.

The drum 2 may be surrounded by a casing 12, indicated by broken lines in Figure l, spaced from the drum and having an air inlet adjacent its bottom and an air outlet adjacent its top (neither being shown), making it a circulating heater. The upright portion of the down draft pipe is outside the casing 12, and the snout 4 also extends beyond it. The back of the drum 2 has an opening 13 near the top, to receive the chimney stack pipe, not shown.

The snout 4 has a secondary air inlet opening 14 in a side thereof, which admits air to the drum 2 at a level above the fuel chamber. The passage of air through this opening is controlled by the thermostatic damper unit, best shown in Figures 2 and 3. Said unit 15 comprises the base flange 16, having a central hole 17 congruent with the secondary air inlet opening 14. Said base flange rests fiat against the planiform side of the snout 4, being held in close contact therewith by the stove bolts 18, which secure the unit to the snout. A bridge piece 19 overlies the opening 17, being bent to form an intermediate rectangular loop 20 otfset from the opening 17 a sufficient distance to constitute a damper chamber, and to form base flanges 21 resting fiat against the base flange 16, and being secured by spot welding. A bimetallic strip 22 is secured to the base plate at one end adjacent an open side of the rectangular loop 20, said bimetallic strip being conventionally curved intermediate its ends, and having a free terminal portion 23 welded or otherwise secured to a disk-like damper 24. The damper operates within the rectangular loop 20 and makes flat contact with the base plate 16 when in closed position. Normally it is biased towards open position through the resiliency of the bimetallic strip, and is adjustable by means of a screw 25 penetrating the transverse portion of the rectangular loop 20, and having its end bearing upon the damper adjacent its center. The thermostat 22, which controls the admission of secondary air is so related to the thermostat which controls the down draft damper that when the former is opening, the latter is closing, and vice versa. In other words, the dampers, both affected by stove temperature, operate in opposite phase.

The operation of the invention is apparent from the foregoing description of the apparatus by means of which it is implemented. As the down draft damper begins to close, reducing the oxygen supplied to the fuel and producing conditions of imperfect combustion which ordinarily would cause an accumulation of unburned gas above the fuel, the secondary air damper begins to admit secondary air to the stove in the zone in which the unburned gas would normally accumulate, increasing the amount admitted progressively and in opposite phase with respect to the progressive closing of the down draft damper. Thus, the gas given off unburned is made combustible in the zone above the fuel chamber so that it burns as fast as it is generated, ignited by flames from the fuel chamber, and does not accumulate unburned, as a potential explosive.

The area of the secondary air inlet opening 14 is much smaller than the cross-sectional area of the down draft opening, since the amount of objectionable unburned gas collecting above the fuel chamber during closed down draft periods, is much less than the quantity of gas consumed during the normal active periods of combustion, and requires proportionately less air.

The small amount of secondary air admitted has negligible influence upon combustion conditions in the combustion chamber.

While we have in the above description disclosed what we believe to be a practical embodiment of the invention, it will be understood by those skilled in the art that the specific details of construction and arrangement of parts are by way of example and not to be construed as limiting the scope of the invention.

What we claim is:

1. Wood burning down draft stove comprising a drum having a surrounding side wall, top and bottom, the lowermost portion of said drum being a fuel chamber,

a down draft pipe communicating with said fuel member adjacent the bottom of said drum providing primary air for normal combustion, the upper part of said drum being provided with an opening for connection to a chimney stack pipe, a damper in the upper part of said down draft pipe and a thermostat so mounted as to respend predominantly to stove temperature for moving said damper in damper closing direction as the stove temperature rises, said down draft pipe when said damper is closed harboring a column of warm air convectionurgcd contra to the stack draft to produce substantially static condition within said drum resulting in dearth of oxygen therein for complete combustion, and accumulation of volatilized fuel constituents in an intermediate zone of said drum, a secondary inlet opening through the wall of said drum in said intermediate zone, a damper for said secondary air inlet opening and a thermostat for controlling the damper associated with said secondary inlet opening mechanically independent of the first mentioned thermostat and also mounted so as to respond predominantly to stove temperature, said thermostats being set to operate said dampers in opposite phase.

2. Wood burning down draft stove as claimed in claim 1, said drum having an access door at an. intermediate point in its height, the sill of which determines the upper level of said combustion chamber.

3. Wood burning down draft stove as claimed in claim 1, the side wall of said drum being provided with an access door opening for fuel having its sill at an intermediate level in the height of said drum, said sill de termining the upper level of said combustion chamber, said side including a snout having a peripheral wall framing said opening and a door closing said snout at the front, said secondary air inlet opening penetrating the side of said snout above the level of said sill.

4. Wood burning down draft stove as claimed in claim 1, the damper for said secondary air inlet opening being part of a unit comprising a base flange secured to said stove about said secondary air inlet opening having a hole therethrough in registry with said opening, a strip secured to said base flange having an offset intermediate portion bridging said hole, said secondary air inlet damper being operative between the plane of said hole and said bridging portion, a bimetallic strip secured at one end to said base flange and at the other to said damper, normally biasing said damper to open position when cold, an adjusting screw through said bridging portion bearing against said damper substantially at its middle.

References Cited in the file of this patent UNITED STATES PATENTS 487,521 Glendinning Dec. 8, 1892 1,157,255 Quinn Oct. 19, 1915 1,910,919 Hetherington May 23, 1933 2,277,515 Harrell Mar. 24, 1942 

